1. Field of the Invention
This invention relates to the electrorefining of lead and, more particularly, to bipolar refining of lead using a fluosilicic acid containing electrolyte.
2. Description of the Prior Art
Electrolytic refining of lead has been proposed using solutions of lead acetate, chloride or nitrate. In United States Pat. No. 209,056, Keith discloses a process for refining impure lead placed as anodes in an electrolyzing solution containing acetate, chloride or nitrate of lead, and made acid by means of either acetic or hydrochloric acid and depositing lead on cathodes. The insoluble impurities such as gold, silver and other metals are retained in muslin bags surrounding the circular anodes. The lead adheres to circular cathode plates in loose crystalline masses. Another early process for electrorefining lead is the Tommasi Process which is described in "The Metallurgy of Lead" by Henry F. Collins, London, Charles Griffin Co. Ltd., 1910, page 452, and which employs a series of disc cathodes revolving between anodes and half immersed in the electrolyte containing a double acetate of lead and potassium. The spongy lead is scraped off the cathode discs mechanically. Neither the Keith nor the Tommasi process proved satisfactory on a commercial scale because of the low output per tank and failure to produce regular deposits at higher current densities, in spite of some improvements obtained through the addition of glue or gelatin.
The first commercial electrolytic refining process for lead was that developed by Betts. This process, known as the Betts Process, is disclosed in U.S. Pat. Nos. 679,824, 713,277 and 713,278 and described in detail in the literature (e.g. see Collins, supra, pages 452-455 and A. G. Betts, "Lead Refining by Electrolysis", John Wiley & Sons, 1908). In the Betts Process, cast anodes of impure lead bullion and pure lead cathode starting sheets are placed in alternate fashion in electrolytic cells containing an aqueous electrolyte comprising hydrofluosilicic acid and lead fluosilicate. Anodes and cathodes are supported on contact bars connected to heavy copper bus bars which are in turn connected to a power source. During electrolysis, refined lead deposits on the cathodes and, while lead dissolves from the anodes, the impurities more noble than lead are left as metallic slimes adhering to the surface of the undissolved portion of the anodes. At the completion of the refining cycle, which may take from 3 to 14 days, the cathodes and anodes are removed from the cells. The cathodes are washed, melted, and cast into shapes for sale. The refined lead has a purity exceeding 99.99%. The wet slimes are separated from the undissolved portion of the anodes, washed to recover electrolyte and treated for the recovery of metal values. The undissolved anode bullion is remelted and recast into anodes. The Betts Process is generally operated at anode voltages not exceeding 0.2 V to avoid dissolution of bismuth, and at current densities in the range of 120 to 220 A/m.sup.2 (amperes per square meter). Addition agents are used to ensure a level deposit of lead on the cathodes and avoid short-circuits between anodes and cathodes. But for relatively minor improvements, the Betts Process has been in use for over seventy years. It is presently the only commercial lead electrorefining process in the world.
Because of mostly economic pressures, it has become necessary to investigate mechanization and automation of the Betts Process. But, although certain improvements can be attained in the process using the multiple system with resultant increased efficiency of operation, the physical lay-out of the process with its separate casting and handling of anodes and cathodes does not particularly lend itself to major improvements. Indeed the structural weakness of the cathode starting sheets makes it difficult to accurately place them in the electrorefining cells.